1. Field of the Invention
The present invention relates to a piezoelectric and/or electrostrictive film-type element. In particular, the present invention relates to a piezoelectric and/or electrostrictive film-type element such as those of the uni-morph type or the bi-morph type for generating or detecting bending displacement or force, to be principally used, for example, for actuators, filters, displays, transformers, microphones, sounding bodies (such as speakers), various vibrators, resonators, oscillators, discriminators, gyroscopes, and sensors. The present invention also relates to a method for producing such a piezoelectric and/or electrostrictive film-type element. The element referred to herein is intended to include elements which convert electric energy into mechanical energy, i.e., mechanical displacement, force, strain, or vibration, as well as elements which perform conversion reversely from the latter to the former.
2. Description of the Related Art
Recently, it has been demanded, in the fields of optics, precision manufacturing, etc., to use a displacement element for adjusting the optical path length or the position on the order of submicron, and a detecting element for detecting minute displacement after converting it into an electric variation. In order to respond to such a demand, there have been developed piezoelectric and/or electrostrictive elements to be used for actuators and sensors, which function as an element to utilize occurrence of displacement based on the reverse or converse piezoelectric effect caused when an electric field is applied to a piezoelectric material such as a ferroelectric substance, or which function as an element to utilize a phenomenon reverse to the foregoing. Among them, as for speakers, for example, those preferably adopted as such a piezoelectric and/or electrostrictive element structure include, for example, those of the uni-morph type which have been hitherto known.
In such a state of art, the present applicant has been also previously proposed piezoelectric and/or electrostrictive film-type elements made of ceramics, which can be preferably used for various applications, as described, for example, in Japanese Laid-Open Patent Publication Nos. 3-128681 and 5-49270. The previously proposed piezoelectric and/or electrostrictive film-type elements have the following structure. Namely, the element comprises a ceramic substrate having at least one window (hollow space) and including a thin-walled diaphragm section provided integrally to cover and close the window so that at least one thin-walled wall section is formed. The element further includes, on an outer surface of the diaphragm section of the ceramic substrate, a piezoelectric and/or electrostrictive orating section comprising a combination of a lower electrode, a piezoelectric and/or electrostrictive layer, and an upper electrode, in which the piezoelectric and/or electrostrictive operating section is integrally stacked and formed in accordance with a film-forming method. The element has such excellent features that it serves as a compact and inexpensive electromechanical conversion element with high reliability to provide a large displacement at a low driving voltage, in which the response speed is quick, and the generated force is large. It is acknowledged that such a piezoelectric and/or electrostrictive film-type element is extremely useful to be used, for example, as a constituting component of actuators, filters, displays, and sensors.
The present inventors have made further investigation on such a piezoelectric and/or electrostrictive film-type element. As a result, the following facts have been clarified. Namely, the piezoelectric and/or electrostrictive film-type element as described above is constructed such that a lower electrode, a piezoelectric and/or electrostrictive layer, and an upper electrode, which construct a piezoelectric and/or electrostrictive operating section, are successively stacked and formed in a layered configuration in accordance with the film-forming method at a predetermined position on a diaphragm section of a fired ceramic substrate, to which a necessary heat treatment (firing) is applied to provide a structure in which the piezoelectric and/or electrostrictive operating section is integrally formed on the diaphragm section. However, the piezoelectric and/or electrostrictive characteristics of the piezoelectric and/or electrostrictive film-type element obtained as described above are considerably degraded depending on the heat treatment (firing) upon the formation of the piezoelectric and/or electrostrictive operating section, specifically the piezoelectric and/or electrostrictive layer.
Namely, when the piezoelectric and/or electrostrictive film-type element as described above is produced, a structure is actually adopted, in which the piezoelectric and/or electrostrictive layer is formed to be slightly larger than the lower electrode so that the lower electrode is covered therewith, in order to avoid a short circuit which would be otherwise formed by the upper electrode formed on the upper surface of the iezoelectric and/or electrostrictive layer and the lower electrode formed on the lower surface of the piezoelectric and/or electrostrictive layer, and well maintain insulation between the upper and lower electrodes. Accordingly, peripheral edge portions of the piezoelectric and/or electrostrictive layer, which extend laterally beyond peripheral edge portions of the lower electrode, are apt to firmly fuse or conglutinate (join) with portions of the diaphragm section of the ceramic substrate located just under the peripheral edge portions, because the peripheral edge portions of the piezoelectric and/or electrostrictive layer are composed of a piezoelectric and/or electrostrictive material which generally contains a Pb component or the like, having high reactivity with other materials. Therefore, such fused or conglutinated portions (joined portions) serve to increase the residual stress on the piezoelectric and/or electrostrictive layer. As a result, the piezoelectric and/or electrostrictive characteristics of the obtained piezoelectric and/or electrostrictive film-type element are degraded.
Further, the following problem has been inherently involved. Namely, the presence or absence of junction, or the dispersion in degree of junction between the peripheral edge portions of the piezoelectric and/or electrostrictive layer extending laterally beyond the lower electrode and the portioins of the diaphragm section located just thereunder evokes, for example, dispersion in sintering of the piezoelectric and/or electrostrictive layer, dispersion in the residual stress, and dispersion in rigidity of the element, among respective piezoelectric and/or electrostrictive film-type elements obtained by production, or among a plurality of piezoelectric and/or electrostrictive operating sections included in one piezoelectric and/or electrostrictive film-type element, resulting in, for example, large dispersion in piezoelectric and/or electrostrictive characteristics such as the amount of displacement, and large dispersion in resonance frequency characteristics.
In such a situation, Japanese Laid-Open Patent Publication No. 6-260694 discloses a piezoelectric and/or electrostrictive film-type element having a structure in which a piezoelectric and/or electrostrictive film on a lower electrode is allowed to have a size sufficient to cover the lower electrode, with its end portions protruding to positions over a ceramic substrate, so that the protruding portions are in an incomplete connection state with respect to the ceramic rate. However, in this patent document, the incomplete connection state is intended to be realized by using the ceramic substrate having low reactivity with respect to the piezoelectric and/or electrostrictive film or by forming a dummy layer comprising a resin material or the like between them. Therefore, this technique involves difficult problems concerning production steps, including, for example, necessity to precisely control conditions for firing the piezoelectric and/or electrostrictive film, such as the firing temperature and the firing atmosphere. Moreover, a problem still remains in reproducibility to repeat the production steps even if adjustment is performed as described above.